1. Field of the Invention
This invention relates generally to the fabrication of giant magnetoresistive (GMR) magnetic field sensors of a “current-perpendicular-to-the-plane” (CPP) configuration. More particularly, it relates to such a sensor that is geometrically patterned, using a single electron beam formed mask and a self-aligned double lift-off scheme, to lower its resistance and redistribute its current in a manner that increases sensor sensitivity and eliminates local hot-spots caused by excessive Joule heating.
2. Description of the Related Art
Magnetic read sensors that utilize the giant magnetoresistive (GMR) effect for their operation are generally of the “current-in-the-plane” (CIP) configuration, wherein current is fed into the structure by leads that are laterally disposed to either side of an active sensor region and the current moves through the structure essentially within the planes of its magnetic and other conducting layers. Since the operation of GMR sensors depends on the detection of resistance variations in their active magnetic layers caused by changes in the relative directions of their magnetic moments, it is important that a substantial portion of the current passes through those layers so that their resistance variations can have a maximally detectable effect. Unfortunately, CIP GMR sensor configurations typically involve layer stacks comprising layers that are electrically conductive but not magnetically active and that play no role in providing resistance variations. As a result, portions of the current are shunted through regions that produce no detectable responses and, thereby, the overall sensitivity of the sensor is adversely affected. The CPP sensor configuration avoids this current shunting problem by disposing its conducting leads vertically above and below the active sensor stack, so that all of the current passes perpendicularly through all of the layers as it goes from the lower to the upper lead. The CPP configuration thereby holds the promise of being effective in reading magnetically recorded media having recording densities exceeding 100 Gbit/in2.
The CPP configuration is not without its problems, however. Whereas current in the CIP configuration passes through parallel conducting layers, in the CPP configuration it passes through such layers in series. The inherent problem in the CIP configuration is the loss of current (and sensitivity) through conductive, but non-magnetic layers; the analogous problem in the CPP configuration is the large voltage drop across magnetically inactive high resistance layers, which tends to mask the voltage variations produced by the active layers. The GMR resistance ratio, DR/R, is typically on the very low order of 1% for the CPP design, because the DR is provided by variations of the low resistance, magnetically active layers, whereas R includes the high resistance of inactive layers. It is worth noting that the high value of R also increases Joule heating in the sensor, causes local hot-spots and, therefore, limits the allowable magnitude of the sensing current.
GMR stack designs favor the use of magnetically pinned layers that are pinned by antiferromagnetic (AFM) pinning layers. Antiferromagnetic materials used in such pinning layers, together with their seed layers, tend to be formed of high-resistance materials and it is these layers that provide a parasitic resistance, Rpa, that is included in R and lowers the sensitivity, DR/R, of the CPP sensor.
One approach to alleviating this problem is to discover and use low-resistance AFM materials. This would necessitate a difficult materials search. An alternative approach is to lower the effective parasitic resistance of the AFM layer and other layers as well by changing their geometry. That is the approach taken by the present invention, particularly as relates to the fabrication of a synthetic spin valve configuration, i.e. a configuration in which the pinned layer comprises a pair of ferromagnetic layers with antiparallel magnetizations coupled by an appropriate material layer formed between them and held in that configuration by a pinning layer of antiferromagnetic material.
The pertinent prior art cited below has offered no similar method for improving the sensitivity of the CPP design having a synthetic spin valve stack configuration. Lederman et al. (U.S. Pat. No. 5,627,704) discloses a CPP GMR stack structure formed within a gap located in one of two pole layers of a magnetic yoke structure which also has a transducing gap formed in an ABS plane. The two pole pieces of the yoke serve to guide magnetic flux to the GMR stack which has current leads above and below it and permanent magnet biasing layers horizontally disposed on either side of it. Sin et al. (U.S. Pat. No. 6,353,318) provides a method for forming a CPP sensor having hard bias layers positioned so as not to allow shorting between the current carrying leads. Dykes et al. (U.S. Pat. No. 5,668,688) discloses a spin valve CPP configuration in which the active layers form a stack of uniform width disposed between upper and lower shield and conductor layers. Stearns et al. (U.S. Pat. No. 6,002,553) discloses a CPP 3-dimensional microarchitecture in which the stack layers are substantially rectangular in shape and of very small size (between 0.1 and 5 microns). Barr et al. (U.S. Pat. No. 6,134,089) discloses a CPP design in which the sensor leads are shaped to have low resistance without the necessity of an increase in spacing between the upper and lower magnetic shields between which the sensor stack is disposed.
The prior art cited above does not discuss or disclose a method of forming a CPP GMR sensor in which the geometry of the various layers permits a re-distribution of current within the sensor stack that effectively reduces its resistance and thereby increases it sensitivity.